1. Field of the Invention
The present invention relates to an ink jet recording apparatus. More particularly, the invention relates to an ink jet recording apparatus capable of obtaining the correct head temperature for the execution of an optimal head driving.
2. Related Background Art
There has been known an ink jet recording method whereby to form images by discharging ink onto a recording medium. The method has the advantages that a recording is possible at a high speed in a high density, and the formation of a color image is easy.
For the ink jet recording method, it is known that the temperature in a printer and the head temperature cause the viscosity of a liquid to change, resulting in the variation of the discharging amount. In other words, the viscosity of the liquid is lowered as the head temperature rises, thus making the discharging amount greater. Regarding changes in the discharging amount by the temperature in apparatus, the discharging amount is small if the temperature in the printer is low, for example, thus making the density of an image low. On the contrary, if the inner temperature is high, the density of an image is high. This difference between the inner temperatures creates a problem of varying the density which is an important element for the formation of good quality images. Here, in order to solve these problems, there has been proposed the means with which to detect the temperatures in apparatus, and prevent the discharging from being lowered even at a low environmental temperature.
At a low environmental temperature, the temperature in apparatus is detected by such a means as above, and then, in order to prevent the discharging amount from being reduced by the temperature in apparatus which is lower than a certain threshold value, a means such as a heat-retaining heater is arranged in a head and controlled to raise the head temperature. In this way, it is possible to correct the head temperature for the elimination of the phenomenon that the density becomes low at the low temperature in apparatus. However, it is still impossible to prevent the discharging amount from being increased in proportion to the temperature rise when the temperature in apparatus becomes higher than the threshold value. The density of an image becomes high due to the temperature rise of the head itself while in printing, which inevitably brings about the difference between densities in a one-page portion or within a line. This problem cannot be solved just by detecting the temperature in apparatus. Also, a specific time is required to correct the head temperature, hence a disadvantage that the throughput is reduced. Moreover, in the proposed method, it is intended to maintain the discharging amount at a desired level just by detecting the temperature in apparatus without detecting the head temperature. This means that while the required temperature parameters for a liquid jet recording apparatus are two, namely, the temperature in apparatus and the head temperature, the head temperature is not detected. As a result, the temperature difference is generated between the temperature in apparatus and the head temperature. A deviation ensues from this difference with respect to the maintenance of the target discharge amounts, thus making the exact control of the discharging amount impossible. Also, in the prior art, a single pulse is given per discharge. With this, the discharging amount cannot be controlled exactly, either.
In this respect, as a method for controlling the discharging amount in an ink jet recording apparatus, there has been proposed the one in which plural pulses are given per ink droplet, such as disclosed in Japanese Patent Laid-Open Application No. 4-247951, U.S. patent application Ser. No. 104,261 (a continuation of U.S. Ser. No. 821,773) and European Patent Laid-Open No. 496,525. The proposed method relates to an ink jet recording which utilizes thermal energy for discharging ink. As a specific example, there has been the one known as thermal jet method which uses the electrothermal transducers arranged in the vicinity of the discharging ports for the generation of the thermal energy in response to pulses, and then, by the thermal energy thus generated, air bubbles are formed in ink for the purpose of discharging it.
FIG. 11 is a view showing an example of such pulses. For the pulses shown in FIG. 11, a pulse P1 supplying the energy which is retained within a range so that no ink discharge is allowed is applied to the electrothermal transducers for the purpose of raising the temperature of ink near the electrothermal transducers. At interval of the time P2 which presents the period during which the pulse is quiescent, a pulse P3 is provided for discharging ink. The temperature of ink near the electrothermal transducers is controlled by changing the thermal energy given by the pulse P1. Thus, the characteristic properties of ink that its viscosity changes according to temperatures are utilized to change the foaming volume by the application of the pulse P3 for discharging ink. In this way, it is possible to control the discharging amount.
Now, by combining this driving method and a method for detecting the head temperature, another method is proposed for the provision of a control in order to improve the image quality. The driving control method is such that the head temperature is detected when the printing signals are inputted, for example, and then, the driving parameters are set to obtain the target discharging amount at the time of the temperature detection. Subsequently, the head temperatures are detected at time intervals which are arbitrarily set during the period of printing so that the driving parameters are modified at any time to match those at which the target discharging amount is obtainable. In this way, it becomes possible to suppress the difference in densities caused by the temperature in a printer and also the difference in densities in a page and in a line due to the temperature rise of the head itself in printing.
Now, to do this control, means for detecting the head temperature must be provided. For example, it is possible to arrange such a means for detecting the head temperature by providing a temperature sensor (such as diodes or aluminum) fabricated on a substrate by a film formation method as in the case of providing the electrothermal transducers in the head. However, when the temperature sensor is formed by the film formation process, the individual difference takes place in the film thickness of each sensor to be formed. This individual difference in the film thickness produces the individual differenc in the resistance value of each temperature sensor. Since the resistance value of the temperature sensor is used for detecting the temperature, the resultant outputs at the same temperature tend to differ from each other (FIG. 13). Nevertheless, although the value of the initial output of each sensor at the same temperature has the individual difference, its temperature-dependent coefficient is constant. The temperature-dependent coefficient is defined as follows: EQU Temperature-dependent coefficient=(Sa-Sb)/(Ta-Tb)
where Sa is the value of sensor output (T=a), Sb is the value of sensor output (T=b), Ta is the temperature=a, and Tb is the temperature=b.
Since the irregularities exist between the individual sensors as described above, the rank classification is arranged according to the prior art as means to correct such irregularities when each of the sensors is prepared. In order to know the absolute temperature using such temperature sensor, the rank classification functions as means to correct the differences in the temperatures obtained by reading the values of sensor outputs corresponding to the initial values of resistance. Then, the following is required for the execution of the rank classification:
At first, the width of the sensor resistance value (width of resistance value in one rank) is set within a range which does not create any problems that may affect images to be formed.
Then, the total range of the individual differences of the sensor resistance values are divided by the width of one-rank resistance value for the preparation of a rank table containing the sensor resistance values and ranks.
The sensor resistance values are measured when the sensors are manufactured, and then, in accordance the above-mentioned table, a pattern cutting is provided for the head to enable the printer main body to discriminate one rank from another by following the pattern cutting. Then, in accordance with the output value and temperature per rank stored in the printer main body, the temperatures are detected. According to the temperatures thus detected, the driving pulses are set. However, in order to classify the ranks of the temperature sensors, the number of processing steps is inevitably increased at the time of manufacture. The production yield is also lowered. Yet, in this respect, not only resistors are needed for the head to execute the pattern cutting, but also a specific circuit is needed for the printer main body to recognize the different ranks. Therefore, the means for detecting the head temperature for the purpose of eliminating the deviation which tends to take place in the temperature in apparatus and the head temperature brings about the significant cost increase eventually.
As described above, according to the prior art, the driving control is given after having detected the temperature in apparatus. However, as the difference is created between the temperature in apparatus and the head temperature, the discharging amount cannot be controlled appropriately, hence leading to the generation of the density differences. Also, as a countermeasure to it, a method is provided to detect the head temperature, but this method necessitates the rank classification of the temperature sensors, which inevitably increases the cost of manufacture and lowers the product yield. Furthermore, for a liquid jet recording apparatus, means for detecting the sensor ranks must be provided as an additional constituent, which also affects the cost in fabricating the apparatus main body.